This invention relates in general to wireless communication systems, and more specifically to a method and apparatus in a two-way radio messaging unit for limiting a total transmission duration within a sliding time window.
The U.S. Federal Communication Commission (FCC) has recently published Guidelines for Human Exposure to Radiofrequency Electromagnetic Fields (OET Bulletin 65, Edition 97-01, August 1997). The Guidelines specify recommended criteria for localized Specific Absorption Rate (SAR) evaluation. For purposes of the Guidelines, a portable device is defined as a transmitting device designed to be used such that the radiating structure(s) of the device is/are within 20 centimeters of the body of the user. For devices which have an inherent property or duty-cycle, the Guidelines allow time-averaging over a thirty-minute sliding time window.
Two-way radio messaging units, such as acknowledge-back pagers, can operate within 20 centimeters of the body of the user. It is thus desirable for the transmitters of two-way messaging units to meet the SAR recommendations of the FCC. Two-way messaging unit transmitters historically have operated at relatively low transmit duty cycles, easily complying with the FCC Guidelines. It is conceivable, however, that some future, high-volume data applications could result in higher transmit duty cycles.
Thus, what is needed is a method and apparatus in a two-way radio messaging unit for limiting a total transmission duration within a sliding time window. Preferably, the method and apparatus will maximize throughput of the two-way messaging unit, while providing full compliance with the Guidelines. 
An aspect of the present invention is a method in a two-way radio messaging unit for limiting a total transmission duration within a sliding time window of a predetermined duration. The two-way radio messaging unit utilizes a protocol in which information is sent in a plurality of periodically transmitted frames. The method comprises the steps of defining a plurality of bins within the sliding time window, each bin for storing a total transmit duration of a predetermined number of frames assigned to each bin; and determining a mean transmit duration per frame for a next one of the plurality of bins to be processed. The method further comprises the steps of subtracting said mean transmit duration per frame from the total transmit duration of the one of the plurality of bins, when a next frame begins to be processed; and calculating a transmit duty cycle for the sliding time window, the transmit duty cycle calculated as if a requested transmit duration for the next frame were to be allowed. The method also includes the step of delaying a transmission of the next frame, when the transmit duty cycle is greater than a maximum allowable transmit duty cycle.
Another aspect of the present invention is a two-way radio messaging unit for limiting a total transmission duration within a sliding time window of a predetermined duration. The two-way radio messaging unit utilizes a protocol in which information is sent in a plurality of periodically transmitted frames. The two-way radio messaging unit comprises a transceiver for receiving and transmitting information, and a processing system coupled to the transceiver for processing the information and controlling the two-way radio messaging unit. The processing system is programmed to define a plurality of bins within the sliding time window, each bin for storing a total transmit duration of a predetermined number of frames assigned to each bin; and to determine a mean transmit duration per frame for a next one of the plurality of bins to be processed. The processing system is further programmed to subtract said mean transmit duration per frame from the total transmit duration of the one of the plurality of bins, when a next frame begins to be processed; and to calculate a transmit duty cycle for the sliding time window, the transmit duty cycle calculated as if a requested transmit duration for the next frame were to be allowed. The processing system is also programmed to delay a transmission of the next frame, when the transmit duty cycle is greater than a maximum allowable transmit duty cycle.
Another aspect of the present invention is an apparatus for use in a two-way messaging unit including a transmitter, the apparatus for limiting a total transmission duration within a sliding time window of a predetermined duration. The apparatus utilizes a protocol in which information is sent in a plurality of periodically transmitted frames. The apparatus comprises a processing system for controlling the two-way messaging unit. The processing system is programmed to define a plurality of bins within the sliding time window, each bin for storing a total transmit duration of a predetermined number of frames assigned to each bin; and to determine a mean transmit duration per frame for a next one of the plurality of bins to be processed. The processing system is further programmed to subtract said mean transmit duration per frame from the total transmit duration of the one of the plurality of bins, when a next frame begins to be processed; and to calculate a transmit duty cycle for the sliding time window, the transmit duty cycle calculated as if a requested transmit duration for the next frame were to be allowed. The processing system is also programmed to delay a transmission of the next frame, when the transmit duty cycle is greater than a maximum allowable transmit duty cycle.